(1) Field of the Invention
The invention relates to demodulation apparatus for demodulating phase-modulated signals and to fibre-optic sensor systems incorporating demodulation apparatus.
(2) Description of the Art
Phase-modulated signals arise in numerous systems, for example in communications systems and sensing systems. Demodulation of such a signal typically involves its combination with an unmodulated signal at a demodulator (e.g. a mixer) to generate a demodulated output signal having an amplitude or magnitude that depends only on the phase of the modulated signal, as in homodyne or heterodyne detection for example. Frequently, the same unmodulated signal is used both to generate the modulated signal (e.g. by passing it to a sensor which carries out phase-modulation in sympathy with conditions in the sensor's vicinity) and for combining with the modulated signal. In a time-division multiplexed (TDM) serial array of fibre-optic sensors, optical pulses from a particular sensor may be selected for analysis by demultiplexing, detected at a photodetector and the resulting signal input to a demodulator together with a unmodulated signal.
One common type of demodulator is the phase-quadrature demodulator which requires input of in-phase (I) and quadrature (Q) unmodulated signals of the substantially the same frequency as the phase-modulated signal in order to provide demodulation. Normally such signals are generated from a single unmodulated signal (for example by use of a Hilbert-transform filter) in which case the means for generating the I and Q signals must be able to adjust the frequency of these signals in response to changes in the frequency of the single unmodulated signal (and hence also that of the modulated signal.) If the speed of adjustment is too slow, the demodulator's output does not consistently reflect the phase of the modulated signal.